1. Field of the Invention
The present invention relates to a method of selecting an operating frequency in a wireless communications system and related communications device, and more particularly to a method of selecting an operating frequency related to multimedia broadcast multicast service, called MBMS, services for a UE in a wireless communications system and related communications device.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. To enhance multimedia functions in the 3G mobile telecommunications system, a protocol specification developed by the 3rd Generation Partnership Project (3GPP) provides Multimedia Broadcast Multicast Service (MBMS). MBMS is a point-to-multipoint bearer service and is established on an existing network architecture of the Universal Mobile Telecommunications System (UMTS), utilizing Internet Protocol (IP) packets as a medium. Thus, MBMS allows a single source entity to transmit data to multiple user equipments (UEs) simultaneously.
According to the protocol specification developed by the 3GPP, MBMS offers two delivery modes: MBMS Broadcast delivery mode and MBMS Multicast delivery mode. When interested in a specific MBMS service, the UE needs to order the MBMS service from a Broadcast Multicast Service Center (BM-SC) by establishing a service agreement including specific service provision phases. These two delivery modes have different service provision phases. The service provision phase of MBMS Broadcast delivery mode includes Service announcement, Session Start, MBMS notification, Data Transfer and Session Stop. The service provision phase of MBMS Multicast delivery mode includes Subscription, Service announcement, Joining, Session Start, MBMS notification, Data Transfer, Session Stop and Leaving. For realizing customized services, Joining and Leaving sessions utilized in MBMS Multicast delivery mode enable the BM-SC to execute authorization and payment recording for the UEs at the Joining session. The service provision phases mentioned above are well known in the art, and will not be described in detail.
From the standpoint of the radio resource control (RRC), all logical data communication exchange channels, be they for providing data transmission exchange to the UE or for providing RRC layer control signal transmission exchange, are defined in the context of a Radio Bearer (RB). In the UE end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels. In the network end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels. For example, a UMTS Terrestrial Radio Access Network (UTRAN) transmits MBMS configuration to the UE by establishing RBs and the UE stops receiving MBMS service by releasing the RBs. According to the RRC protocol specification provide by the 3GPP, an RRC state of the UE can be a disconnection mode or an RRC connection mode. The disconnection mode includes Idle mode, and the RRC connection mode includes CELL_PCH (Cell Paging Channel), URA_PCH (UTRAN Registration Area Paging Channel), CELL_FACH (Cell Forward Access Channel) and CELL_DCH (Cell Dedicated Channel) states.
The MBMS services include two types: Broadcast and Multicast types. Generally, the Broadcast type services are free so that the UE does not need to perform the Joining session before receiving them. For Broadcast type services, the UTRAN always adopts the broadcast transmission mode using point-to-multipoint (p-t-m) RBs to transmit service data and configuration. On the other hand, for the Multicast type services, the UTRAN determines whether to utilize the broadcast or multicast transmission mode based on the service charge policy. In the multicast transmission mode, the UE needs to perform the Joining session, whereas in the broadcast transmission mode, the UE does not need to. In addition, the UTRAN performs a counting procedure to obtain the number of the UEs interested in receiving MBMS services and thereby determines to use point-to-point (p-t-p) or p-t-m RBs. In the following, a transmission via the p-t-p RBs is regarded as a p-t-p transfer mode, and a transmission via the p-t-m RBs is regarded as a p-t-m transfer mode.
The UTRAN generally includes two or more carrier frequencies for providing a variety of services for UEs, and the UEs are appropriately distributed on these carrier frequencies. The UTRAN may provide MBMS services only on one of the carrier frequencies to avoid same MBMS data stream being transmitted on the different frequency layers. Furthermore, the UTRAN may transmit an MBMS MODIFIED SERVICES INFORMATION message to notify the UE of transition to a cell of the preferred frequency layer for the concerned MBMS services. After the frequency transition, the UE will store the previous frequency which the UE operates at.
At the Session Stop of an MBMS service, the UTRAN transmits an MBMS required UE action Information Element (IE) included in the MBMS MODIFIED SERVICES INFORMATION message to the UE in Idle mode, CELL_PCH state, URA PCH state or CELL_FACH state under the p-t-m transfer mode. By setting the MBMS required UE action IE to ‘Release PTM RB’, the UTRAN can notify the UE of the Session Stop, and thereby the UE releases corresponding p-t-m RBs to stop receiving the MBMS service. In addition, if the MBMS MODIFIED SERVICES INFORMATION message also includes an MBMS dispersion indicator IE, the UE will return to the cell of the stored previous frequency if the UE does not decide to receive another MBMS services on the current frequency. Thus, a notification of the Session Stop transmitted by the UTRAN can direct the UE to transit out of the preferred frequency layer.
Additionally, the UTRAN periodically transmits an MBMS GENERAL INFORMATION message to notify the UE of the current use condition of the preferred frequency layer. For example, if the MBMS GENERAL INFORMATION message includes an MBMS PL (Preferred Layer) Service Restriction Information IE, the UTRAN will not provide any non-MBMS Services (e.g. a mobile-terminated call service). This also indicates that the preferred frequency layer is congested. On the contrary, if the MBMS GENERAL INFORMATION message does not include the MBMS PL Service Restriction Information IE, there are still free radio resources available on the preferred frequency layer where the UTRAN can also provide the non-MBMS Services.
To enhance the MBMS for mobile TV delivery, the 3GPP introduces a concept of MBMS Selected Services, which are a subset of MBMS activated services of the Broadcast type, for which the UE applies RRC procedures to inform the UTRAN which MBMS services have been selected by upper layers in the UE. An MBMS_ACTIVATED_SERVICES variable, related to the MBMS activated services, stores information about the MBMS Multicast services the UE has joined, as well as the MBMS Broadcast services the UE is interested in receiving. The stored information may include Activated service list, Service type, etc. According to the prior art, the MBMS Selected Services, especially the Mobile TV services, are assumed to be available all the time. Hence, there will be no Session Stop notification from the UTRAN and the reception of the MBMS Selected Services will be stopped by the UE. Moreover, according to the RRC specification of 3GPP, if the reception of any MBMS services is stopped by the UE in the p-t-m transfer mode, the UE in Idle mode, CELL_PCH state, URA PCH state or CELL_FACH state only releases related p-t-m RBs without considering returning to the previous frequency layer which the UE operates at. As a result, all the UEs receiving the MBMS Selected Services will stay on the preferred frequency layer after stopping the service reception.
Besides the UEs receiving the MBMS Selected Services, the new UEs, which are not on the preferred frequency layer and decide to receive the MBMS services, will enter the preferred frequency layer. Therefore, the preferred frequency layer is liable to get congested due to increase of the UE number. If the preferred frequency layer is congested, the UE staying on this layer will not be able to receive any new non-MBMS services, such as the mobile-terminated call service.